Gas hydrate inhibitors

ABSTRACT

The present invention relates to a method to inhibit gas hydrate formation in the field of crude oil and natural gas extraction, transportation and processing.

FIELD OF THE INVENTION

The present invention relates to a method to inhibit gas hydrateformation in the field of oil and natural gas extraction, transportationand processing.

STATE OF THE ART

Gas hydrates (or clathrate hydrates, gas clathrates, clathrates, etc.)are crystalline water-based solids physically resembling ice, in whichsmall non-polar hydrocarbon molecules (typically gases) are trappedinside “cages” of hydrogen bonded water molecules. In other words, gashydrates are clathrate compounds in which the host molecule is water andthe guest molecule is typically a hydrocarbon gas.

Gas hydrates cause problems for the petroleum industry because they canform solid crystals inside oil/gas pipelines, transfer lines, valves andother equipment. Since they have also a strong tendency to agglomerateand to adhere to the pipeline walls, the formation of gas hydrates mayeven result in obstructions of the pipelines. Preventing gas hydrateformation is therefore desirable in the art of producing, transportingand processing crude oil and natural gas.

One method to control the growth of gas hydrates is by employingchemicals that can lower the hydrate formation temperature and/or delaytheir formation (gas hydrate inhibitors). Different kinds of gas hydrateinhibitors exist: thermodynamic inhibitors and kineticinhibitors/anti-agglomerants.

The most common thermodynamic inhibitors are lower alkyl alcohols andglycols.

Kinetic inhibitors and anti-agglomerants are also known asLow-Dosage-Hydrate-Inhibitors (LDHI), because they require much smallerconcentrations than the conventional thermodynamic inhibitors. Whilekinetic inhibitors act by slowing down the kinetics of the nucleation,anti-agglomerants prevent the agglomeration (self adhesion) of gashydrate crystals. Kinetic inhibitors are usually synthetic polymers orcopolymers, while anti-agglomerants are often quaternary ammoniumcompounds (R₁R₂R₃R₄N⁺A⁻ where all of R₁, R₂, R₃ and R₄ are organicgroups and A⁻ is an anion) having surface active properties. Thesequaternary ammonium compounds and trialkyl amine salts with varioussubstituents are described in many patents, such as in U.S. Pat. Nos.5,460,728, 5,648,575, 6,214,091 (Shell Oil Company, US), U.S. Pat. No.6,595,911 (Baker Hughes Inc., US), U.S. Pat. No. 7,381,689 (ChampionTechnologies, Inc.), U.S. Pat. No. 8,034,748 (Clariant Produkte, DE)

In particular, U.S. Pat. No. 7,381,689 discloses a method of controllinggas hydrate blockage through the addition, among the others, of amino orquaternary ammonium amide salts of formula:

where: A is N; R₁, R₂, R₄, and R₅ are organic moieties; R₁ is an alkylhaving from 4 to 5 carbon atoms; R₂ is hydrogen or an alkyl having from1 to 4 carbon atoms; R₄ is —(CH2)_(t)-, wherein t is an integer 2 to 4;R₅ is an organic moiety, for example an alkyl or alkenyl group, having 4to 20 carbon atoms; X⁻ is an anion; and a is 0 or 1. When a is 1, thenR₃ is selected from hydrogen, organic moieties having from 1 to 20carbon atoms, and combinations thereof.

The X⁻ anion can be selected from hydroxide, carboxylate, halide, suchas chloride e bromide, sulfate, organic sulphonate, and combinationsthereof, but only bromide ammonium salts are really disclosed.

These quaternary ammonium amide bromides, have many advantages: theyperform well at very low dosages, may be prepared from largelyavailable, highly reactive, low cost, raw materials, such as alkyl andalkenyl bromide. Moreover they are ecologically friendly: in fact theyare easily biodegraded in alkaline environments and exhibit low fishtoxicity upon degradation. Unfortunately, quaternary ammonium halideshave some drawbacks too.

First of all, quaternary ammonium halides undergo thermal decomposition.Two types of decomposition reactions usually take place simultaneously:the removal of one of the N-alk(en)yl groups as an alk(en)yl halide withformation of tertiary amines, and elimination of hydrogen halide throughextraction of an hydrogen atom from one of the N-alk(en)yl groups withformation of mixture of tertiary amine halide salts and olefin. Althoughtertiary amine salts have been described as being effective as LDHI too,the unselective thermal decomposition often leads to low performingmixtures of compounds.

Moreover, halide ions in the presence of water are potentially damagingto metals because they may lead to the formation of hydro halogenic acidand to its accumulation. This can be an enormous problem in a field inwhich metal equipments constantly come into contact with water or withoil/water (possibly acidic) two-phase systems. This is particularly truefor equipments which were not built in stainless steel or an high alloysteel or which were not treated for resisting to corrosive fluids, suchas brines or seawater. Drums, transfer-lines, valves, tanks andinjection systems, which are used for the storage, the preparation andthe addition of the additives, are examples of these equipments.

The absence or the almost complete reduction of halide ions and organichalides in additives that are used at producing sites, pipelines ortanks is therefore highly desirable in order to mitigate corrosionproblems.

Finally, quaternary ammonium amides, also known as amidoquats, arenotorious surfactants and they are used in many field as foaming andemulsifying agents.

In the field of producing, transporting and processing crude oil, notonly foaming is a problem which can slow down and reduce the efficiencyof the processes, but also it can reduce the drainage, i.e. separationof water from the oil phase.

In addition, in the presence of a surfactant, oil and aqueous fluids mayform emulsions that undesirably increase the viscosity of the mixtureand thereby increase the power required to transport the oil. Moreover,the produced hydrocarbons and the aqueous fluids must generally beseparated, and where an emulsion has formed such separation may be verydifficult.

It is an object of the present invention to provide a gas hydrateinhibitor based on a quaternary ammonium amide salt which do not containhalides and have very little tendency to metal corrosion and to stablefoam/emulsion formation in comparison with the gas hydrate inhibitors ofthe prior art.

Now, it has been surprisingly found that using an alkyl sulfate or alkylcarbonate or carbonate salt of a quaternary ammonium amide with arelatively short fatty chain, it is possible to obtain an effective gashydrate inhibition without the above mentioned problems.

As far as the Applicant knows, the use of these salts as gas hydrateinhibitors have never been described before.

DESCRIPTION OF THE INVENTION

It is, therefore, an object of the present invention, a method forinhibiting formation of gas hydrates in systems comprising mixture ofhydrocarbons and water, said method comprising the addition to themixture of a quaternary ammonium amide salt of formula I:

whereinR₁(CO)— is the residue of a saturated or unsaturated, linear orbranched, aliphatic carboxylic acid containing from 6 to 24, preferablyfrom 6 to 20, more preferably from 8 to 18, carbon atoms;R₂ and R₃ are, independently of each other, a butyl or a pentyl group;R₄ is linear or branched alkyl group having from 1 to 5, preferably from2 to 4, more preferably 2 or 3, carbon atoms;X can be R₄OSO₃ ⁻, R₄O(CO)O⁻, bicarbonate and carbonate;a can be 1 or 2;with the proviso that at least 50% by weight of the aliphatic carboxylicacid contains less than 16 carbon atoms.

DETAILED DESCRIPTION OF THE INVENTION

Preferably, R₁(CO)— is the residue of a saturated or unsaturated, linearor branched aliphatic carboxylic acid wherein at least 60% by weight ofsaid acid contains less than 16 carbon atoms.

In a preferred embodiment of the method of the invention, R₂ and R₃ arethe same and are a butyl group.

The quaternary ammonium amide salt of formula I of this invention can beprepared by quaternization of a tertiary amino amide of formula II:

wherein R₁, R₂ and R₃ have the same meaning as reported above.

The tertiary amino amide of formula II can be obtained by condensationof a saturated or unsaturated, linear or branched aliphatic carboxylicacid having formula R₁COOH and a N,N-substituted propylene diamine offormula R₂R₃N—CH₂CH₂CH₂—NH₂, wherein R₂ and R₃ are, independently ofeach other, a butyl or pentyl group.

Specific examples of saturated or unsaturated, linear or branchedaliphatic carboxylic acids, suitable for the realization of the presentinvention, are hexanoic acid, 2-ethyl hexanoic acid, n-octanoic acid,n-nonanoic acid, n-decanoic acid, n-undecanoic acid, n-dodecanoic acid,myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleicacid, linoleic acid, linolenic acid and the like.

Also mixtures of saturated or unsaturated, linear or branched aliphaticcarboxylic acids can be used for the realization of the presentinvention. Suitable examples are mixtures of carboxylic acids derivedfrom natural oils, such as coco fatty acids, palm kernel fatty acids andpalm fatty acids.

Preferred aliphatic carboxylic acids are mixtures of fatty acids fromnatural oil and particularly preferred are palm kernel fatty acids andcoco fatty acids, the latter being the most preferred.

The preferred N,N-substituted propylene diamine is N,N-dibutyl propylenediamine.

The preparation of carboxylic acid amides of substituted diamines iswell known in the art. In an exemplary preparation process, theN,N-substituted propylene diamine is reacted with about 0.95 to about1.1 molar equivalents of the carboxylic acid, ester or acid chloride, ata temperature ranging from about 110 to about 220° C. for about 1 toabout 30 hours.

The process of preparation of the quaternary ammonium amide salt offormula I requires a further reaction step wherein the amino groupspresent in the molecule are substantially all quaternized.Quaternization is a reaction type that is well known in the art:typically it contemplates the reaction of a substrate with an alkylatingagent.

For the quaternization step of the present invention, the alkylatingagent can be selected from the group consisting of dialkyl sulfate anddialkyl carbonate, wherein the alkyl group has from 1 to 5 carbon atoms.Specific examples of these alkylating agents are dimethyl sulfate,diethyl sulfate, dimethyl carbonate and diethyl carbonate, dipropylsulfate, etc. The most preferred alkylating agents being diethyl sulfateand diethyl carbonate.

In one embodiment of the present invention, the tertiary amino amide offormula II is melt or dissolved in a suitable solvent, such as a C₁-C₄alcohol or diol, and quaternized with about 0.95 to about 1.5 molarequivalents of a dialkyl sulfate to form the quaternized ammonium amidesalt. The temperature is normally comprised between 70 and 140° C.Isopropanol, propylene glycol and methanol are the preferred solventsfor the quaternization as they exhibit the best ability at reducing theviscosity of the quaternary salt solution. Isopropanol and propyleneglycol are preferred over methanol because of the toxicity issuesassociated with use of methanol.

The aforementioned quaternary ammonium amide salts of alkyl carbonates,carbonates and bicarbonates can be prepared by methods known in the art,such as those described in U.S. Pat. No. 5,438,034 and WO 03/006419.

It must be pointed out that the quaternary ammonium amide carbonates andbicarbonates of the invention are in equilibrium. The ratiobicarbonates/carbonates varies depending on the pH of the solution inwhich they are contained.

In one embodiment, the method of the present invention comprises theaddition to the mixture of hydrocarbons and water of the quaternaryammonium amide salt as such, without any further diluents or additives.

In another embodiment, the method of the present invention comprises theaddition of a gas hydrate inhibitor composition, comprising thequaternary ammonium amide salt as herein described, a solvent (e.g. aliquid solvent) and other optional additives.

The gas hydrate inhibitor composition of the invention can comprisebetween 20 and 95% by weight, preferably between 45 and 90% by weight,more preferably between 55 and 85% by weight, of the quaternary ammoniumamide salt.

Representative solvents suitable for formulation with gas hydrateinhibitor include polar solvents such as water, alcohols (includingstraight chain or branched aliphatic alcohols such as methanol, ethanol,2-ethoxyethanol, propanol, isopropanol, butanol, isobutanol, hexanol),glycols and glycol ether derivatives (including ethylene glycol,propylene glycol, hexylene glycol, ethylene glycol monobutyl ether,ethylene glycol dibutyl ether, or diethylene glycol monomethyl ether),ethers (e.g., tetrahydrofuran), amides (e.g., N-methyl-2-pyrrolidinoneor N,N-dimethylformamide), ketones (e.g. methyl ethyl ketone,cyclohexanone, or diisobutyl ketone); apolar solvents, such as aromatichydrocarbon solvents (e.g. toluene and xylene); and mixtures thereof.

Preferred solvents are methanol, propylene glycol and isopropanol.

Suitable optional additives are paraffin inhibitors, asphalteneinhibitors, scale inhibitors, corrosion inhibitors, oxygen scavengers,hydrogen sulfide scavengers, non emulsifiers and emulsion breakers.

The quaternary ammonium amide salts, according to the present invention,are particularly suitable as gas-hydrate inhibitors when added tohydrocarbon fluids containing water.

They may be used by simple addition to the hydrocarbon fluids to betreated.

In the preferred procedure of this invention, the quaternary ammoniumamide salts are added to a flowing hydrocarbon fluid which may containboth oil and water, at any point in a flow line upstream of the point orline that is intended to be protected. The dosage of gas hydrateinhibitor of the invention needed to obtain a sufficient protectionvaries with the application, but it is advantageously added in such anamount that the concentration is between 0.1 and 8.0% by weight,preferably between 0.5 and 5.0% by weight and more preferably between1.0 and 3.5% by weight.

Examples Gas Hydrate Inhibition Test

The performances of the gas hydrate inhibitors of the invention wereevaluated with a Rocking Cell RC5 by PSL Systemtechnik.

Test Fluids

-   -   Oil Phase: Diesel    -   Aqueous Phase: 4% wt seasalt water or deionized water    -   Gas: Mix of methane, ethane, propane and butane (various        isomers)

Test Procedure

The sapphire test cells, containing a stainless steel ball, were filledwith the fluids (see Table 1) and 2% by weight of inhibitor andpressurized with the gas mixture.

Inhibitor Fluid (v/v) Test 1 2% 50/50 4% wt seasalt water/diesel Test 22% 20/80 deionized water/diesel

Each cell was the subjected to a cycle of cooling and rocking consistingof three steps: 1) flowing condition, 2) shut-in and 3) re-start flowingcondition.

-   1) The pressurized cells were cooled down to 4° C. over a period of    5 hours while rocking. After reaching 4° C., the cells were rocked    for 12 hours.-   2) The rocking was stopped and the test cells were kept at 4° C. in    horizontal position (shut-in) for 16 hours.-   3) At the end of the shut-in period, rocking was re-started for 4    hours. Finally the cells were heated back to room temperature.

At the beginning of the third step, the content of the cells wasvisually evaluated.

Each cycle was replicated three times and the results registered.

Results

The results of the gas hydrate inhibition tests are reported in Table 2according to the following scale:

-   -   FAIL: The ball is stuck and/or large agglomerations and/or solid        crystals and/or visible deposits on the cell walls.    -   PASS: The ball is free; solid crystals might be present, but        agglomerates (large or small) break up under rocking.

Test 1 Test 2 benzylcocodimethyl ammonium chloride* FAIL FAILN,N-dibutyl-N-ethyl-cocoamidopropyl PASS PASS ammonium ethyl sulfate*Comparative

Foaming Power Tests

The foam volume (FV) and the foam stability (FS) were determined bystirring for 30 seconds at high speed (8000 rpm) with a Waring Blender100 mL of a 1% by weight solution of the inhibitors in deionized water(Test 3) or in a 4% sea salt water solution (Test 4). The foamedcomposition was then immediately transferred into a graded cylinder forthe determination of the foam volume and the stability of the foam.

FV represent the volume in mL of foam at the end of the stirring. FS isthe time in seconds required to the foamed solution to regenerate 50 mLof liquid. The longer the time, the higher the stability of the foam.

Table 3 shows the results of the foaming power test.

Test 3 Test 4 FV FS FV FS benzylcocodimethyl ammonium chloride* 480 260430 215 N,N-dibutyl-N-ethyl-soyamidopropyl 330 185 230 116 ammoniumethyl sulfate* N,N-dibutyl-N-ethyl-cocoamidopropyl 230 104 165 58ammonium chloride* N,N-dibutyl-N-ethyl-cocoamidopropyl 190 65 150 44ammonium ethyl sulfate *Comparative

The results demonstrate that the quaternary ammonium amide salts offormula I of the invention produce less foam than ammonium salts of theprior art.

Corrosion Tests

The Linear Polarization Resistance (LPR) measurements were made with aGamry Electrochemical Instrument system.

The LPR corrosion tests were conducted in 1 L Pyrex jacketed cells. 900mL of synthetic brine (50/50 v/v 4.0% Seasalt water/Fresh Water) wereloaded in the cell placed on a magnetic stirrer, deaerated overnightwith CO₂ and, finally, saturated with 200 ppm H₂S gas just beforetesting. A clean C1018 Mild Steel rod was inserted in the corrosion cellassembly as sample probe.

A graphite rod was used as the counter electrode. The temperature of thesolution was brought to 80° C. for the duration of the tests and CO₂ wascontinuously purged at a constant flow rate. The gas hydrate inhibitorswere added at 10 ppm by volume of test solution.

The results are reported in Table 4 as % of protection after a fixedperiod of time compared to the blank, the test solution without anyinhibitor, which is considered 100% corrosion.

% Protection 1 hour 16 hours benzylcocodimethyl ammonium chloride* 79.789.5 N,N-dibutyl-N-ethyl-cocoamidopropyl 82.8 91.9 ammonium chloride*N,N-dibutyl-N-Etyl-cocoamidopropyl 86.1 94.1 ammonium ethyl sulfate*Comparative

The results demonstrate that the quaternary ammonium amide salts offormula I of the invention produce less corrosion than ammonium chloridesalts of the prior art.

1. A method for inhibiting the formation of gas hydrates in systems comprising mixtures of hydrocarbons and water, said method comprising adding to the mixture of a quaternary ammonium amide salt of formula I:

wherein R₁(CO)— is the residue of a saturated or unsaturated, linear or branched, aliphatic carboxylic acid containing from 6 to 24 carbon atoms; R₂ and R₃ are, independently of each other, a butyl or a pentyl group; R₄ is linear or branched alkyl group having from 1 to 5 carbon atoms; X can be R₄SO₄ ⁻, R₄O(CO)O⁻, bicarbonate and carbonate; a can be 1 or 2; with the proviso that at least 50% of the aliphatic carboxylic acid contains less than 16 carbon atoms.
 2. The method of claim 1, wherein, in the quaternary ammonium amide salt of formula I, R₁(CO)— is the residue of a saturated or unsaturated, linear or branched aliphatic carboxylic acid wherein at least 60% by weight of said acid contain less than 16 carbon atoms.
 3. The method of claim 1, wherein, in the quaternary ammonium amide salt of formula I, R₂ and R₃ are the same and are a butyl group.
 4. The method of claim 1, wherein, in the quaternary ammonium amide salt of formula I, R₄ is linear or branched alkyl group having from 2 to 4 carbon atoms.
 5. The method of claim 4, wherein R₄ is linear or branched alkyl group having 2 or 3 carbon atoms.
 6. The method of claim 1, comprising adding to the mixture of hydrocarbons and water of the quaternary ammonium amide salt of formula I as a composition comprising between 20 and 95% by weight of said salt, a solvent and other optional additives.
 7. The method of claim 6, wherein the composition comprises between 45 and 90% by weight of said quaternary ammonium amide salt.
 8. The method of claim 1, comprising adding to the mixture of hydrocarbons and water of between 0.1 and 8.0% by weight of quaternary ammonium amide salt of formula I.
 9. The method of claim 8, comprising of adding between 0.5 and 5.0% by weight of said quaternary ammonium amide salt. 